1. Field of the Invention
The invention relates to an X-ray system, comprising an X-ray tube and an X-ray generator for operating the X-ray tube which comprises a cathode which can be heated by a filament current, comprising means which are operative in an exposure mode so as to boost the filament current to a boost value, and means which are also operative in the exposure mode so as to decrease the filament current and to switch on the tube voltage.
2. Description of the Related Art
When such an X-ray system is used to form an X-ray image, for example after prior fluoroscopy, it is desirable to execute the X-ray exposure as quickly as possible. In the case of X-ray tubes comprising a heatable cathode, however, the cathode (or the filament contained therein) must first be heated to a temperature at which it can emit the tube current required for the X-ray exposure.
In order to reduce the time elapsing until the start of exposure it is known to supply the cathode, while the tube voltage is still switched off, with a filament current which is substantially larger than the filament current required for the subsequent X-ray exposure (with the tube voltage switched on). The boost time is governed by the tube current required to flow during the subsequent exposure. The larger this tube current, the longer the boost time will be.
It is already known to store the boost times required for a given type of tube in an X-ray generator in a memory and to fetch these boost times for an X-ray exposure. Such a boost time table in the memory is compiled by the manufacturer of the X-ray tube by way of a complex measuring procedure which is performed separately for each type of X-ray tube. The boost times thus defined are typical values, i.e. it may occur that the cathode temperature at the end of the boost time is higher or lower than the temperature required for the relevant tube current. Therefore, after the boost time the filament current is reduced to the value required for the X-ray exposure. When the tube voltage is switched on after a further time interval of from 200 to 300 ms, the cathode temperature has reached a stationary value which corresponds to the value required for the exposure.